Information on the snow water equivalent (SWE) of the snowpack is essential to agencies involved in water management, such as power and energy producers like Hydro-Québec. Typically, in Quebec, the snowpack cumulated at the end of winter represents annually some 30% of the total available hydraulic energy. It is also important to detect when the snowpack begin to melt, and from there on, to measure the melting rate. Those data are also of significant importance for other applications or concerns like civil safety (early flood warnings).
One traditional way of assessing the SWE of a snowpack is by boring the snowpack along established snow courses and manually taking measurements. The main disadvantages of those manual measurements are their repetitive costs and inaccuracy especially during the snow melting period. In winter, a high proportion of the snow courses may be attended to only by helicopters, which is an expensive operation. This limits the frequency of the snowpack measurements over a given ground area. Boring samples out of the wet snowpack provides unreliable information, which usually underestimates the SWE. Thus, real time and accurate data on the SWE and on the soil moisture (SM) content especially in the case of frozen soil underneath the snowpack are still a challenge today in spite of many initiatives to develop and test alternative techniques and equipments. Harsh climatic conditions prevailing over northern regions is an additional problem with respect to the equipments.
Snow pillow sensors have been used extensively by the Natural Resources Conservation Service (NRCS). Most of their snow survey sites are located throughout the western United States, as reported in Snotel and Scan, P. Pasteris, NOAA Snowfall Network Observation Workshop, Kansas City, Mo., June 2004. California's water resources depend on the snowmelt-dominated Sierra Nevada and snowmelt rates are measured using snow pillows in the Yosemite National Park as reported by Lundquist and al. in Meteorology and Hydrology in Yosemite National Park: A Sensor Network Application, Springer Berlin/Heidelberg, Lecture Notes in Computer Science, Volume 2634/2003, pp. 518-528. As snow accumulates or melts at the top of the pillow filled with a liquid glycol solution, a pressure is measured and correlated to the SWE. However, operation costs are high for the maintenance of this kind of sensors and their size makes them difficult to install in off-road locations like northern sites in Quebec. De-icing the snow pillows is sometimes a necessary maintenance operation to obtain non bias SWE data.
U.S. Pat. No. 6,313,645 (Brandelik et al.) discloses a method for the determination of the volumetric proportion of liquid water, the thickness of the snowpack and the density of snow. The dielectric coefficient of the snow is measured using a probe consisting of at least three electric conductors. An advantage of this method over previous works based on the dielectric properties of the snow is that measurements are taking place without influences of an air gap which is always present between the instrument cables and the snow and could vary with weather conditions in the range of 0.5 to 3 mm. Two pairs of cables are combined into a single three-wire cable for real determination of the dielectric constant of the snow. However, relating the information on the dielectric coefficient to SWE is difficult since snow presents different behaviours according to its physical properties such as the shape of crystals, the temperature, etc. To be reliable, this technique therefore requires additional information and calibration of the snowpack.
Techniques based on radioactivity measurements are also known for the SWE measurement. One radioactive technique is based on the attenuation of secondary background cosmic gamma radiation. U.S. Pat. No. 5,594,250 (Condevra) mentions that the preferred energy range is 3 to 10 MeV which is a good compromise between the ability of the device to determine the SWE and the size of the detector. Energies below 2.7 MeV include counts from terrestrial background gamma sources, which in this case are discarded as noise and are then not desired by this device. The relative measurement of the gamma ray attenuation by snow is exclusively due to its water content and not due to the air entrapped. A first detector is placed directly at ground level, monitors the variations in cosmic radiation in relation with the depth and characteristic of the snowpack. A second detector, above the snowpack, monitors the variations in cosmic rays unaffected by snow. The simultaneous measurements of the two detectors are compared to derive information on the snowpack.
The need of a second detector increases cost despite the fact that this second detector can be shared between a number of ground detectors at nearby sites. However, a second detector contiguous to the one installed near the ground may be useful to allow anticoincident exclusion of false readings due to the high energy primary cosmic radiation. In U.S. Pat. No. 6,663,012 (Condevra), the attenuation of secondary cosmic radiation in the range of 5-15 MeV is used to detect the soil moisture.
U.S. Pat. No. 4,047,042 (Wada et al.) and U.S. Pat. No. 4,992,667 (Abelentsev et al.) describe similar approaches with devices for measuring moisture content of soil and snow water storage using two neutron detectors. A first neutron detector is positioned at a pre-set depth in the soil and a second neutron detector is positioned at an altitude greater than a maximum snowpack thickness. A gamma radiation detector is needed to subtract the background signal from the soil. The operation of the devices is relatively complex and large deployment is limited possibly due to cost.
The use of artificial radioisotope source radiation like 60Co to determine the water content of soil and/or the snowpack is also known. For example, Canadian patent No. 1,079,413 (Morrison) describes a precipitation gauge where the radioactive source is put on the ground and detectors are placed above the ground. In the case of U.S. Pat. No. 3,432,656 (Smith et al.), the approach is inverted and consists of placing the artificial source above the maximum snowpack thickness with a radiation detector installed at a preset depth in the soil. Some disadvantages from these approaches are: that relatively large artificial sources are needed and must be free of regulatory constraints or qualified for general licensing; the necessity to provide biological protection to the operators; and the pollution of the environment and the possible disappearance via vandalism. Those serious drawbacks discredit such an approach for large deployment because organizations are now more concerned with the protection of the environment. U.S. Pat. No. 4,614,870 (Morrison) describes the use of small artificial radioisotope sources to detect water content but it has limitations in penetration distance. This method is mainly used for measuring moisture content in several discrete zones of different materials. The use of artificial source for SWE determination has been initially developed a few decades ago and U.S. Pat. No. 3,432,656 (Smith et al.) and U.S. Pat. No. 3,665,180 (Guillot et al.) are among those describing first applications. One interesting feature implemented in the device described by Smith et al. is the possibility of moving the source and the detector to determine the SWE of a larger volume of snow.
Flying large gamma detectors at low altitude over extensive lines (5 kilometers long or more) is another alternative. Initially developed in Russia, as reported by A. V. Dmitriev et al. in Fundamentals of remote methods for measuring snow water storage and moisture content of soil using gamma-radiation of the Earth, 1979 Gidrometeoizdat Publishing House, Leningrad, pp. 281-288, it has been implemented in various countries, including Canada as reported by Grasty et al. in An experimental gamma-ray spectrometer snow survey over Southern Ontario, US/IHD Interdisciplinary symposium on advanced concepts and techniques in the study of snow and ice resources, Monterey, California, Dec. 2-6, 1973, pp. 6.1-1 to 6.1-16, and United States as reported by Carroll et al. in B.E., 1993. A comparison of U.S. and Canadian Airbone Gamma Radiation Snow Water Equivalent Measurements, Western Snow Conference 1983, pp. 27-37. This technique is still in operation over regions of these countries. The main drawbacks of the technique are the complexity of the method and the calibration procedure, the requirement for dedicated and sophisticated equipment including the aircraft, and its high cost. This limits the frequency of the surveys over a specific region.